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Mendel's Genetics
For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids. It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown.  Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction.  The focus of genetics research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children.  A number of hypotheses were suggested to explain heredity, but Gregor Mendel  was the only one who got it more or less right.  His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death.  His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brno. In his later years, he became the abbot of his monastery and put aside his scientific work.

While Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms.

Through the selective cross-breeding of common pea plants over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.  For instance, the pea flowers are either purple or white--intermediate colors do not appear in the offspring of cross-pollinated pea plants.  Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

	flower color is purple or white	5	seed color is yellow or green
	flower position is axil or terminal	6	pod shape is inflated or constricted
	stem length is long or short	7	pod color is yellow or green
	seed shape is round or wrinkled

This observation that these traits do not show up in offspring plants with intermediate forms was critically important because the leading theory in biology at the time was that inherited traits blend from generation to generation.  Most of the leading scientists in the 19th century accepted this "blending theory."  Charles Darwin proposed another equally wrong theory known as "pangenesis".  This held that hereditary "particles" in our bodies are affected by the things we do during our lifetime.  These modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation.  This was essentially a variation of Lamarck's incorrect idea of the "inheritance of acquired characteristics."

Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated.  Pea plants have both male and female reproductive organs.  As a result, they can either self-pollinate themselves or cross-pollinate with another plant.  In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.  This was the basis for his conclusions about the nature of genetic inheritance. 

In cross-pollinating plants that either produce yellow or green pea seeds exclusively, Mendel found that the first offspring generation  always has yellow seeds.   However, the following generation  consistently has a 3:1 ratio of yellow to green.

This 3:1 ratio occurs in later generations as well.   Mendel realized that this underlying regularity was the key to understanding the basic mechanisms of inheritance.

He came to three important conclusions from these experimental results:

1.	that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged      (these units are now called genes)
2.	that an individual inherits one such unit from each parent for each trait
3.	that a trait may not show up in an individual but can still be passed on to the next generation.

It is important to realize that the starting parent plants were homozygous for pea seed color.  That is to say, they each had two identical forms of the gene for this trait--2 yellows or 2 greens.  The plants in the f1 generation were all heterozygous.   In other words, they each had inherited two different alleles--one from each parent plant.  It becomes clearer when we look at the actual genetic makeup, or genotype, of the pea plants instead of only the phenotype, or observable physical characteristics.

Note that each of the f1 generation plants inherited a Y allele from one parent and a G allele from the other.  When the f1 plants breed, each has an equal chance of passing on either Y or G alleles to each offspring.

With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other, which is to say it masked the presence of the other allele.  For example, when the genotype for pea seed color is YG (heterozygous), the phenotype is yellow.  However, the dominant yellow allele does not alter the recessive green one in any way.   Both alleles can be passed on to the next generation unchanged.

Mendel's observations from these experiments can be summarized in two principles:

1	the principle of segregation
2	the principle of independent assortment

According to the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.  Which allele in a parent's pair of alleles is inherited is a matter of chance.  We now know that this segregation of alleles occurs during the process of sex cell formation (i.e., meiosis).

According to the principle of  independent assortment, different pairs of alleles are passed to offspring independently of each other.  The result is that new combinations of genes present in neither parent are possible.  For example, a pea plant's inheritance of the ability to produce purple flowers instead of white ones does not make it more likely that it will also inherit the ability to produce yellow pea seeds in contrast to green ones.  Likewise, the principle of independent assortment explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand.  Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes.

These two principles of inheritance, along with the understanding of unit inheritance and dominance, were the beginnings of our modern science of genetics.  However, Mendel did not realize that there are exceptions to these rules.  Some of these exceptions will be explored in the third section of this tutorial and in the Synthetic Theory of Evolution tutorial.

By focusing on Mendel as the father of genetics, modern biology often forgets that his experimental results also disproved Lamarck's theory of the inheritance of acquired characteristics described in the Early Theories of Evolution tutorial.  Mendel rarely gets credit for this because his work remained essentially unknown until long after Lamarck's ideas were widely rejected as being improbable.

Reproduction, process by which organisms replicate themselves.

In a general sense reproduction is one of the most important concepts in biology: it means making a copy, a likeness, and thereby providing for the continued existence of species. Although reproduction is often considered solely in terms of the production of offspring in animals and plants, the more general meaning has far greater significance to living organisms. To appreciate this fact, the origin of life and the evolution of organisms must be considered. One of the first characteristics of life that emerged in primeval times must have been the ability of some primitive chemical system to make copies of itself.

At its lowest level, therefore, reproduction is chemical replication. As evolution progressed, cells of successively higher levels of complexity must have arisen, and it was absolutely essential that they had the ability to make likenesses of themselves. In unicellular organisms, the ability of one cell to reproduce itself means the reproduction of a new individual; in multicellular organisms, however, it means growth and regeneration. Multicellular organisms also reproduce in the strict sense of the term—that is, they make copies of themselves in the form of offspring—but they do so in a variety of ways, many involving complex organs and elaborate hormonal mechanisms.

Levels of reproduction

Molecular replication

The characteristics that an organism inherits are largely stored in cells as genetic information in very long molecules of deoxyribonucleic acid (DNA). In 1953 it was established that DNA molecules consist of two complementary strands, each of which can make copies of the other. The strands are like two sides of a ladder that has been twisted along its length in the shape of a double spring. The rungs, which join the two sides of the ladder, are made up of two terminal bases. There are four bases in DNA: thymine, cytosine, adenine, and guanine. In the middle of each rung a base from one strand of DNA is linked by a hydrogen bond to a base of the other strand. But they can pair only in certain ways: adenine always pairs with thymine, and guanine with cytosine. This is why one strand of DNA is considered complementary to the other.

The double helices duplicate themselves by separating at one place between the two strands and becoming progressively unattached. As one strand separates from the other, each acquires new complementary bases until eventually each strand becomes a new double helix with a new complementary strand to replace the original one. Because adenine always falls in place opposite thymine and guanine opposite cytosine, the process is called a template replication—one strand serves as the mold for the other. It should be added that the steps involving the duplication of DNA do not occur spontaneously; they require catalysts in the form of enzymes that promote the replication process.

Molecular reproduction

The sequence of bases in a DNA molecule serves as a code by which genetic information is stored. Using this code, the DNA synthesizes one strand of ribonucleic acid (RNA), a substance that is so similar structurally to DNA that it is also formed by template replication of DNA. RNA serves as a messenger for carrying the genetic code to those places in the cell where proteins are manufactured. The way in which the messenger RNA is translated into specific proteins is a remarkable and complex process. (For more detailed information concerning DNA, RNA, and the genetic code, see the articles nucleic acid and heredity: Chromosomes and genes). The ability to synthesize enzymes and other proteins enables the organism to make any substance that existed in a previous generation. Proteins are reproduced directly; however, such other substances as carbohydrates, fats, and other organic molecules found in cells are produced by a series of enzyme-controlled chemical reactions, each enzyme being derived originally from DNA through messenger RNA. It is because all of the organic constituents made by organisms are derived ultimately from DNA that molecules in organisms are reproduced exactly by each successive generation.

The chemical constituents of cytoplasm (that part of the cell outside the nucleus) are not resynthesized from DNA every time a cell divides. This is because each of the two daughter cells formed during cell division usually inherits about half of the cellular material from the mother cell (see cell: Cell division and growth), and is important because the presence of essential enzymes enables DNA to replicate even before it has made the enzymes necessary to do so.

Cells of higher organisms contain complex structures, and each time a cell divides the structures must be duplicated. The method of duplication varies for each structure, and in some cases the mechanism is still uncertain. One striking and important phenomenon is the formation of a new membrane. Cell membranes, although they are very thin and appear to have a simple form and structure, contain many enzymes and are sites of great metabolic activity. This applies not only to the membrane that surrounds the cell but to all the membranes within the cell. New membranes, which seem to form rapidly, are indistinguishable from old ones.

Thus, the formation of a new cell involves the further synthesis of many constituents that were present in the parent cell. This means that all of the information and materials necessary for a cell to reproduce itself must be supplied by the cellular constituents and the DNA inherited from the parent cell.

Genetics is the study of the function and behavior of genes. Genes, the basic units of heredity, are biochemical instructions composed of DNA (deoxyribonucleic acid) and are found inside the cells of every organism, from bacteria to humans. An organism’s genes, which reside in one or more chromosomes, determine its characteristics (traits). The sum of all an organism’s genes is called its genome. It is divided into chromosomes, chromosomes contain genes, and genes are made of DNA.

Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. They also study how tiny variations in genes can disrupt an organism’s development or cause disease.

Classical genetics, which remains a basis for all other topics in genetics, primarily is concerned with the method by which genetic traits are transmitted in plants and animals. These traits are classified as dominant (always expressed), recessive (subordinate to a dominant trait), intermediate (partially expressed) or polygenic (due to multiple genes). It  began with Austrian monk Gregor Mendel, who traced the inheritance patterns of certain traits in pea plants and showed they could be described mathematically.

Population, quantitative and ecological genetics build on classical genetics. Population genetics studies the distribution of and change in the frequencies of genes under the influence of evolutionary forces, such as natural selection, mutations and migration. Quantitative genetics is the study of continuous traits (such as height or weight) that do not have straightforward Mendelian inheritance because they result from the interaction of many different genes. Ecological genetics again builds on the basic principles of population genetics but is focused more explicitly on ecological issues, such as the relationship between species and their environments. Medical genetics encompasses many different individual fields, including clinical genetics (the diagnosis and treatment of genetic diseases), cytogenetics (the study of chromosomes under a microscope), molecular genetics and genetic counseling (education and guidance offered by professional advisors to help people make informed decisions based on personal genetic information). Behavioral genetics examines the role of genetics in animal behavior. Genomics examines large-scale genetic patterns across the genome for a given species.

6 Methods of Studying Human Genetics

Some of the basic methods of study of human genetics are as follows:

1. Pedigree records are well- recorded and well maintained so it has become easier to trace the transmission of particular character through generation.

2. Population genetics has been used extensively in the study of human genetics. So the methods by which fate of characters in population can be analysed, has overcome the limitation of small number of progeny in human.

3. Biochemical genetics, cell culture technique and somatic cell genetic techniques have helped to understand the chemical bases of inheritance of large number of characters.

4. Cell fusion technology helps to produce a variety of combinations of human genetic material in the progeny clones. By studying these clones, geneticists have been able to take a fresh approach to human genetics. With this technology it is possible to isolate the hybrids formed between the cells derived from mouse and man.

5. By comparing the phenotypes of identical and fraternal twins (study of twins), the hereditary basis of a number of characters has been established. Heritable and environmentally induced traits can be by comparing comparative study of identical and fraternal twins.

6. In human cytology, new techniques of staining techniques have been developed. With such advanced techniques correct human diploid chromosome number i.e., 46 has been found to be correct.